Prepare the Gelato Base:

1. In a small saucepan set over medium-high heat, heat 1 cup cream and sugar until wisps of steam start to rise from the surface.
2. In a small mixing bowl, whisk together the cornstarch with the remaining 1/2 cup cream. Whisk into the hot cream, then reduce heat to low and simmer for 5 to 6 minutes, whisking constantly, or until the mixture thickens enough to coat the back of a spoon. Remove from heat and immediately stir in the buttermilk, lemon juice, lemon zest, salt and limoncello (if using).
3. Transfer the custard into a mixing bowl and cover with plastic wrap, resting the wrap directly on the surface of the custard to prevent a skin from forming. Move to the refrigerator and chill for at least 2 hours (or preferably overnight).

Make the Raspberry Swirl:

1. In small saucepan set over medium-high heat, stir together raspberries and sugar. Bring to a simmer and cook, stirring occasionally, for about 10 minutes or until the raspberries break down and thicken. Strain through fine mesh sieve to remove the seeds. Stir in the vodka, then cover and chill until ready to assemble the gelato.

Freeze and Assemble the Gelato:

1. Using an ice cream maker, freeze the chilled custard as per the manufacturer's directions.
2. Spoon alternating layers of ice cream and raspberry sauce into a freezer-safe container, then run a skewer or butter knife through the mixture to create a swirl. Transfer to the freezer to chill until firm but scoopable, about 4 hours.